1. Field of the Invention
This invention relates to the oxidation of propylene and is more particularly concerned with the preparation of propylene oxide by using thallic salts.
2. Statement of the Prior Art
Olefins have been oxidized with thallium triacetate in aqueous acetic acid to glycols and carbonyl compounds and in acetic acid and methanol to diacetate and ethers as well as allylic oxidation products. Exemplary of this art is U.S. Pat. No. 3,048,636 (issued in 1962 to R. R. Grinstead) in which a process is described for reaction of the olefin with at least an equimolar quantity of a thallium salt (chloride, nitrate, sulfate, or acetate) in acid solution in a solvent such as water, acetic acid, propionic acid, tetrahydrofuran, or methanol. But under these conditions the product of oxidation consists of a mixture formed solely of glycol and aldehyde or ketones. In the presence of a carboxylic acid, the carboxylic ester of the glycol will also be formed. The reaction, which may be carried out in favor of glycol class products as well as in the favor of the carbonylated products, will generally give rise to quite low yields of oxidation products.
U.S. Pat. No. 3,452,047 (issued in 1969 to A. H. Frye) also describes the oxidation of olefins by using a thallium salt, such as a nitrate or sulfate, in aqueous acid solution. This patent especially indicates that the utilization of a quantity of thallium salt lower by 10 to 90% than equimolecular quantity leads to preferential formation of aldehyde or ketone from the olefin.
W. Kruse et al., J. Org. Chem. 36 1154 (1971) describe the preparation of certain selected epoxides by the oxidation of the corresponding olefin with thallic acetate in weakly solvating media. Carbonyl by-products in the epoxidation of propylene in 50% (v/v) aqueous acetic acid were present in a propylene oxide to acetone molar ratio of 1:1, which ratio was improved to only about 4.5:1 by a use of a less polar solvent, tetrahydrofuran.
U.S. Pat. No. 3,641,067 (issued in 1972 to W. M. Kruse) also describes the preparation of the epoxide of propylene and isobutylene by means of lower alkyl thallic carboxylates, in the presence of a water-miscible organic solvent and an aliphatic monocarboxylic acid containing one to four carbon atoms, and describes use of pressures of up to 30 psi. Again propylene to acetone molar ratios in the products were found to be on the order of about 4.4:1.
Japanese Patent Publication No. 49-13104 (published in 1974) describes a process for converting C.sub.3 to C.sub.5 olefins to the corresponding epoxide wherein an aqueous aliphatic acid solution containing (1) a thallous salt, (2) an alkali or alkaline earth metal salt, and optionally a copper or iron salt of a fatty acid (acetic or propionic acid) or halide, and/or (3) an aliphatic carboxylic acid (acetic acid, propionic acid, etc.), is contacted with air or oxygen to oxidize the solution. The resulting oxidized solution is contacted with the olefin to form the desired epoxide by means of the thallic salt formed in the first step. Alternatively, the olefin may be present in the solution being oxidized so that a one-step process results.
This process is very difficult to control and the oxidation step is severely inefficient in conversion of a thallous salt to the thallic state required for subsequent oxidation of an olefin to the epoxide. Moreover, the lower aliphatic carboxylic acids employed did not cure the problems associated with previous processes as to the ratio of propylene oxide and by-product carbonyl compounds in the product mixture.
French Pat. No. 2,255,298 (issued in 1975 to Societe des Usines Chimiques Rhone-Poulenc) sought to improve the concentration of propylene epoxide in the product and to minimize the formation of acetone by a multi-step process of reacting propylene at a low temperature with an aqueous solution having a pH of not greater than 6 and containing a thallic salt of an aliphatic monocarboxylic acid having from one to four carbon atoms, a carboxylic acid which is miscible with or soluble in water, and a water-miscible organic solvent. (Aliphatic monocarboxylic acids of one to four carbon atoms are specified as being suitable.) The pH of the resulting reaction mixture is next critically adjusted to a value of at least 7 and the adjusted solution then heated, with immediate and rapid removal of the propylene oxide formed.
While French Pat. No. 2,255,298 achieves propylene oxide to acetone molar ratios of from about 10:1 to 16:1, the careful pH control required in this process is quite costly on an industrial scale.
U.S. Pat. No. 4,021,453 to William F. Brill discusses a process for preparing epoxides from the corresponding olefins by means of aryl thallic carboxylates having up to 12 carbon atoms in the presence of an inert polar organic solvent and in the presence of water, and optionally in the presence of free aryl carboxylic acid. The aryl thallic carboxylate is employed alone or in combination with a thallic alkanoate, with a ratio of aryl and non-aryl carboxylate being greater than 1:1, and can be formed in situ as by reacting the corresponding aryl carboxylic acid with thallic acetate.